Business and Basis of Presentation (Policies)	6 Months Ended
Business and Basis of Presentation (Policies)	Jun. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The unaudited interim consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”) and, in the opinion of management, reflect all adjustments that are of a normal recurring nature, necessary for a fair statement of the financial condition, results of operations and cash flows of the Company and its consolidated variable interest entities (“VIEs”) for the periods presented. The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates. These unaudited interim consolidated financial statements are as of June 30, 2015 and cover the three-month period ended June 30, 2015 ("Second Quarter 2015"), the three-month period ended June 30, 2014 ("Second Quarter 2014"), six-month period ended June 30, 2015 ("Six Months 2015") and the six-month period ended June 30, 2014 ("Six Months 2014"). Certain financial information that is normally included in annual financial statements prepared in accordance with GAAP, but is not required for interim reporting purposes, has been condensed or omitted. The year-end balance sheet data was derived from audited financial statements.
Consolidation	The unaudited interim consolidated financial statements include the accounts of AGL, its direct and indirect subsidiaries (collectively, the “Subsidiaries”), and its consolidated VIEs. Intercompany accounts and transactions between and among all consolidated entities have been eliminated.
Future Application of Accounting Standards	Future Application of Accounting Standards Consolidation In February 2015, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2015-02, Consolidation (Topic 810): Amendments to the Consolidation Analysis, which is intended to improve certain areas of consolidation guidance for legal entities such as limited partnerships, limited liability companies, and securitization structures. The ASU will be effective on January 1, 2016. Early adoption is permitted, including adoption in an interim period. The Company does not expect that ASU 2015-02 will have any material effect on its Consolidated Financial Statements. Interest In April 2015, the FASB issued ASU No. 2015-03, Interest - Imputation of Interest (Topic 835-30): Simplifying the Presentation of Debt Issuance Costs, which requires that debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, consistent with debt discounts. The ASU will be effective on January 1, 2016 and should be applied retrospectively. The adoption of this ASU will require the Company to reclassify its debt issuance costs from other assets to long-term debt on the Consolidated Balance Sheet.
Business Combinations	Radian Asset Acquisition was accounted for under the acquisition method of accounting which required that the assets and liabilities acquired be recorded at fair value. The Company was required to exercise significant judgment to determine the fair value of the assets it acquired and liabilities it assumed in the Radian Asset Acquisition. The most significant of these determinations related to the valuation of Radian Asset's financial guaranty insurance and credit derivative contracts. On an aggregate basis, Radian Asset’s contractual premiums for financial guaranty contracts were less than the premiums a market participant of similar credit quality would demand to acquire those contracts at the Acquisition Date, particularly for below-investment-grade transactions, resulting in a significant amount of the purchase price being allocated to these contracts. For information on the methodology the Company used to measure the fair value of assets it acquired and liabilities it assumed in the Radian Asset Acquisition, including financial guaranty insurance and credit derivative contracts, please refer to Note 8, Fair Value Measurement. The fair value of the Company's stand-ready obligation on the Acquisition Date is recorded in unearned premium reserve. At the Acquisition Date, the fair value of each financial guaranty contract acquired was in excess of the expected losses for each contract and therefore no explicit loss reserves were recorded on the Acquisition Date. Instead, loss reserves and loss and loss adjustment expenses ("LAE") will be recorded when the expected losses for each contract exceeds the remaining unearned premium reserve, in accordance with the Company's accounting policy described in the Annual Report on Form 10-K. The expected losses acquired by the Company as part of the Radian Asset Acquisition are included in the description of expected losses to be paid under Note 6, Expected Losses to be Paid. The excess of the fair value of net assets acquired over the consideration transferred was recorded as a bargain purchase gain in "bargain purchase gain and settlement of pre-existing relationships" in net income.
Fair Value of Financial Instruments	The Company carries a significant portion of its assets and liabilities at fair value. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date (i.e., exit price). The price represents the price available in the principal market for the asset or liability. If there is no principal market, then the price is based on a hypothetical market that maximizes the value received for an asset or minimizes the amount paid for a liability (i.e., the most advantageous market). Fair value is based on quoted market prices, where available. If listed prices or quotes are not available, fair value is based on either internally developed models that primarily use, as inputs, market-based or independently sourced market parameters, including but not limited to yield curves, interest rates and debt prices or with the assistance of an independent third-party using a discounted cash flow approach and the third party’s proprietary pricing models. In addition to market information, models also incorporate transaction details, such as maturity of the instrument and contractual features designed to reduce the Company’s credit exposure, such as collateral rights as applicable. Valuation adjustments may be made to ensure that financial instruments are recorded at fair value. These adjustments include amounts to reflect counterparty credit quality, the Company’s creditworthiness and constraints on liquidity. As markets and products develop and the pricing for certain products becomes more or less transparent, the Company may refine its methodologies and assumptions. During Six Months 2015, no changes were made to the Company’s valuation models that had or are expected to have, a material impact on the Company’s consolidated balance sheets or statements of operations and comprehensive income. The Company’s methods for calculating fair value produce a fair value that may not be indicative of net realizable value or reflective of future fair values. The use of different methodologies or assumptions to determine fair value of certain financial instruments could result in a different estimate of fair value at the reporting date. The fair value hierarchy is determined based on whether the inputs to valuation techniques used to measure fair value are observable or unobservable. Observable inputs reflect market data obtained from independent sources, while unobservable inputs reflect Company estimates of market assumptions. The fair value hierarchy prioritizes model inputs into three broad levels as follows, with Level 1 being the highest and Level 3 the lowest. An asset or liability’s categorization within the fair value hierarchy is based on the lowest level of significant input to its valuation. Level 1—Quoted prices for identical instruments in active markets. The Company generally defines an active market as a market in which trading occurs at significant volumes. Active markets generally are more liquid and have a lower bid-ask spread than an inactive market. Level 2—Quoted prices for similar instruments in active markets; quoted prices for identical or similar instruments in markets that are not active; and observable inputs other than quoted prices, such as interest rates or yield curves and other inputs derived from or corroborated by observable market inputs. Level 3—Model derived valuations in which one or more significant inputs or significant value drivers are unobservable. Financial instruments are considered Level 3 when their values are determined using pricing models, discounted cash flow methodologies or similar techniques and at least one significant model assumption or input is unobservable. Level 3 financial instruments also include those for which the determination of fair value requires significant management judgment or estimation. Transfers between Levels 1, 2 and 3 are recognized at the end of the period when the transfer occurs. The Company reviews the classification between Levels 1, 2 and 3 quarterly to determine whether a transfer is necessary.
Consolidation, Variable Interest Entity	As part of the terms of its financial guaranty contracts, the Company obtains certain protective rights with respect to the VIE that are triggered by the occurrence of certain events, such as failure to be in compliance with a covenant due to poor deal performance or a deterioration in a servicer or collateral manager's financial condition. At deal inception, the Company typically is not deemed to control a VIE; however, once a trigger event occurs, the Company's control of the VIE typically increases. The Company continuously evaluates its power to direct the activities that most significantly impact the economic performance of VIEs that have debt obligations insured by the Company and, accordingly, where the Company is obligated to absorb VIE losses or receive benefits that could potentially be significant to the VIE. The Company obtains protective rights under its insurance contracts that give the Company additional controls over a VIE if there is either deterioration of deal performance or in the financial health of the deal servicer. The Company is deemed to be the control party for certain VIEs under GAAP, typically when its protective rights give it the power to both terminate and replace the deal servicer, which are characteristics specific to the Company's financial guaranty contracts. If the protective rights that could make the Company the control party have not been triggered, then the VIE is not consolidated. If the Company is deemed no longer to have those protective rights, the transaction is deconsolidated.

Disclosure of accounting policy for consolidation to describe the significant judgments and assumptions made in determining whether a variable interest held by the entity requires the variable interest entity to be consolidated and (or) disclose information about its involvement with the variable interest entity; the methodology used by the entity for determining whether or not it is the primary beneficiary of the variable interest entity; and the significant factors considered and judgments made in determining that the power to direct the activities that significantly impact the economic performance of the variable interest entity are shared (as defined).